Methods and apparatus described herein are useful in imaging apparatus having post-imaging finishing devices. The term xe2x80x9cimaging apparatusxe2x80x9d includes devices such as printers, photocopies and facsimile machines, which form an image on one or more sheets of imaging media (as for example, paper, transparencies, cardstock, envelopes, etc.). A post-imaging finishing device is a device which works in conjunction with the imaging apparatus to apply xe2x80x9cfinishingxe2x80x9d to the sheets of imaged media. The post-imaging finishing device can be integral with the imaging apparatus, or it can be separate device which can be attached to the imaging apparatus, or placed in imaging-media communication with the imaging apparatus (i.e., imaged sheets of imaging media from the imaging apparatus can be provided to the finishing device for post-imaging finishing). The finishing device can include one or more post-imaging finishing units. A common finishing unit is a stapler (or xe2x80x9cstapling unitxe2x80x9d) which staples together sheets of imaging media that have been deposited into a stack (the xe2x80x9csheet stackxe2x80x9d). Other examples of finishing units include: a sheet binding unit which adheres the sheets of imaged media in a sheet stack to one another together along a common edge of the sheet stack by applying a glue or resin along the edge; a stitching unit which stitches together the sheets of imaged media in a sheet stack along a common edge; and a hole punch unit that punches one or more holes in the sheets of a sheet stack. The operation performed by a finishing unit in a finishing device will be known herein as a xe2x80x9cpost-imaging finishing processxe2x80x9d or a xe2x80x9cfinishing processxe2x80x9d.
Many post-imaging finishing processes (stapling, binding, stitching, hole-punching, etc.) are limited by the thickness of the sheet stack, such that the process cannot or should not be applied to the sheet stack once the stack exceeds a certain thickness. This limitation typically is based on the capacity of the finishing unit. For example, in a stapling unit the limitation can be set by the height of a the staple (or staples) available to the stapling unit, such that the staple is of insufficient length to pass through all of the sheets in the sheet stack and still have sufficient excess length to cleat-over on the last page of the stack (thus binding the stack together into a cohesive stapled set). Further, a stapling unit limitation can be set by the cross-sectional area, and/or the material of fabrication, of the available staple(s), such that a staple will tend to buckle when driven into a sheet stack of more than a certain thickness. A stapling unit limitation can also be set by the power available to the stapler, such that the stapler may have insufficient power to drive a staple through the entire sheet stack. Similarly, in a stitching unit and/or a hole-punch unit, the limitation of the unit can be set by the power available to the unit, such that there is insufficient power for the respective awl and/or punch to penetrate all pages in the sheet stack. In an edge binding unit, glue is applied by an applicator of a certain height, and in this instance the limitation can be set by that height, such that the height of the applicator is insufficient to include all of the sheets in the sheet stack.
When a post imaging finish process is attempted to be provided to a sheet stack which exceeds the capacity of the finishing unit, deleterious results can occur. In the simplest case, the finishing process is not applied to all of the sheets in the sheet stack, in which case a user can choose to either accept an undesirable product, or must reapply or complete the finishing process by other means. Furthermore, attempting to apply a finishing process to a sheet stack when the thickness of the stack exceeds the capacity of the finishing unit can result in a damaged sheet stack, as for example can occur when a staple buckles in the sheet stack. In more serious cases, the finishing unit itself can be damaged when attempting to apply a finishing process to a sheet stack when the thickness of the stack exceeds the capacity of the finishing unit.
One prior art method of estimating the thickness of a sheet stack is to count the number of sheets placed in the sheet stack using a sheet counter (a device that counts sheets as they exit the imaging section of an imaging apparatus and are placed in the output tray). When the counted number of sheets exceeds a pre-set number, the finishing unit can be disabled, since it will be anticipated that more sheets that the pre-set number will exceed the capacity of the finishing unit. However, this method is based on a preselected paper thickness, which is typically the thickness of the thickest paper likely to be encountered. The method suffers from the fact that the thickness of the sheets are not always the same as the estimated sheet thickness. For example, an imaging apparatus can be configured to process sheets having industry-standard weights of from 18 pounds to 32 pounds. (Paper xe2x80x9cweightxe2x80x9d is based on 500 sheets of the paper, each sheet having a width of 17 inches and a length of 22 inches.) In general, paper thickness is proportional to paper xe2x80x9cweightxe2x80x9d, such that a sheet of 32-pound paper will be about 78% thicker than a sheet of 18-pound paper. Accordingly, the method is configured to anticipate that the heaviest (thickest) paper will be used. When thinner paper than the heaviest anticipated paper is used, the finishing unit will be disabled when, in fact, the capacity of the finishing unit (based on the estimated overall thickness of the sheet stack) has not yet been exceeded.
One non-limiting embodiment of the present invention provides for a finishing device which is configured to receive sheets of imaging media forming a sheet stack. The finishing device can receive the sheets from an imaging apparatus, for example. The finishing device includes a sheet stack tray to support the sheet stack, and a sheet stack hold-down device which is operable from a first position to a variable second position. When the hold-down device is in the second position it presses the sheet stack against the sheet stack tray. The finishing device further includes a sensor which can detect the position of the sheet stack hold-down device when it is in the second variable position, thereby providing an approximation of the thickness of the sheet stack based on the actual second position of the hold-down device.
Another embodiment of the invention provides for a method of controlling operation of a finishing unit. The method includes providing a sheet stack made up of sheets of imaging media, and holding the sheet stack against a surface with a sheet stack hold-down device. The position of the sheet stack hold-down device is detected, and the detected position of the sheet stack hold-down device is then used to control operation of the finishing unit.
These and other aspects and embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein: